paint_node: Fix paint_node_damage_below

This list walk is broken, the intent was to walk the tail of the list
starting from the currently held node - but that is not what happens.
Instead, walk the list backwards and stop a the held node.

Also, paint_node_damage_below() is used when moving paint nodes between
planes, and in these cases we definitely don't want to limit damage to
the current plane.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>